1. Field
The present invention relates generally to data communication, and more specifically to techniques for performing selective channel inversion per eigenmode for MIMO systems.
2. Background
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels is also referred to as a spatial subchannel or eigenmode of the MIMO channel.
The spatial subchannels of a wideband MIMO system may encounter different channel conditions due to various factors such as fading and multipath. Each spatial subchannel may thus experience frequency selective fading, which is characterized by different channel gains at different frequencies of the overall system bandwidth. Assuming no power control, this then results in different signal-to-noise-and-interference ratios (SNRs) at different frequencies of each spatial subchannel, which would then be able to support different data rates for a particular level of performance (e.g., 1% packet error rate).
To combat frequency selective fading in a wideband channel, orthogonal frequency division multiplexing (OFDM) may be used to effectively partition the overall system bandwidth into a number of (NF) subbands, which are also referred to as frequency bins or subchannels. With OFDM, each subband is associated with a respective subcarrier upon which data may be modulated. For a MIMO system that utilizes OFDM (i.e., a MIMO-OFDM system), each subband of each spatial subchannel may be viewed as an independent transmission channel.
A key challenge in a coded communication system is the selection of the appropriate data rates and coding and modulation schemes to use for a data transmission based on the channel conditions. A major goal for the system is to maximize spectral efficiency while reducing complexity for both the transmitter and receiver.
One straightforward technique for selecting data rates and coding and modulation schemes is to “bit load” each transmission channel in the system according to its transmission capability. However, this technique has several major drawbacks. First, coding and modulating individually for each transmission channel can significantly increase the complexity of the processing at both the transmitter and receiver. Second, coding individually for each transmission channel may greatly increase coding and decoding delay.
There is, therefore, a need in the art for techniques to achieve high spectral efficiency in MIMO systems without having to individually code for each transmission channel.